In the development of radio communication systems, in particular cellular communication (like for example GSM (Global System for Mobile Communication), GPRS (General Packet Radio Service), UMTS (Universal Mobile Telecommunication System) or the like), efforts are made for an evolution of the radio access part thereof. In this regard, the evolution of radio access networks (like for example the GSM EDGE radio access network (GERAN) and the Universal Terrestrial Radio Access Network (UTRAN) or the like) is currently addressed. Such improved radio access networks are sometimes denoted as evolved or advanced radio access networks (like for example the Evolved Universal Terrestrial Radio Access Network (E-UTRAN)) or as being part of a long-term evolution (LTE) or LTE-Advanced, also generally referred to as International Mobile Communications-Advanced (IMT-A). Although such denominations primarily stem from 3GPP (Third Generation Partnership Project) terminology, the usage thereof hereinafter does not limit the respective description to 3GPP technology, but generally refers to any kind of radio access evolution irrespective of the underlying system architecture.
In the following, for the sake of intelligibility, LTE (Long-Term Evolution according to 3GPP terminology) or LTE-Advanced is taken as a non-limiting example for a broadband radio access network of cellular type being applicable in the context of the present invention and its embodiments. However, it is to be noted that any kind of radio access network of cellular type may likewise be applicable, as long as it exhibits comparable features and characteristics as described hereinafter.
In the development of cellular systems in general, and access networks in particular, a direct communication of mobile cellular devices without involvement of the fixed network part has been proposed as one concept. This concept may be equally referred to as device-to-device (D2D) communications, mobile-to-mobile (M2M) communications, machine-to-machine (M2M) communications, terminal-to-terminal (T2T) communications, peer-to-peer (P2P) communications, or the like. In the following, for the sake of intelligibility, the term device-to-device (D2D) communications is used for this concept, while the term “device” is to be understood as being synonymous with the terms “terminal”, “peer”, “mobile”, or the like.
The motivation and purpose of incorporating direct D2D communications into a cellular network is, among various aspects, to reduce transmitter power consumption in both the device and the network, to improve the spectrum efficiency and network resource utilization, to increase cellular network capacity and coverage, to create and support more services for the users in most efficient fashion, and so on.
In general, mobile cellular D2D communication, D2D for short, is assumed to use licensed radio spectrum under supervision and control of supporting cellular systems. D2D communications may or may not use radio resources of the hosting (i.e. supporting) and/or local cellular system or systems, wherein the former is referred to as in-band D2D in which D2D connections share the radio resources of the cellular system with conventional radio links via base stations such as eNBs, and the latter is referred to as out-band D2D in which D2D connections have available specifically dedicated resources independent of those of the cellular system.
Not only, but particularly in an in-band D2D system where the same radio resources of the supporting cellular system are also used for direct D2D communications, the D2D resource allocation should preferably be under the control of the cellular system e.g. to avoid severe interference between cellular and D2D users. However, the regular resource allocation, monitoring and control mechanisms in cellular networks are not designed for localized and distributed D2D communications in which user data transmission as well as control signaling—at least in part—happen directly between two or multiple mobile devices such as UEs.
Furthermore, as mobile devices are not a trusted entity for an operator's network, a threat of unauthorized usage of radio resources could be created or aggravated when allowing (more) UE-centric distributed control, i.e. mobile devices making (more) decision in conducting direct D2D communications. Therefore, the cellular system shall preferably retain supervision and control of D2D communications both in control signaling as well as in actual user data transfer.
For example, certain control and assistance for D2D resource allocation is necessary and important from network operator's point of view. This raises a problem in how to allow the network to manage and identify distributed D2D users in fast and efficient way upon D2D registration, namely during D2D resource and/or connection establishment.
There exists a need and problem of initial resource allocation for supporting direct D2D communications in cellular networks, i.e. resource allocation for prior to actual D2D communications.
In this regard, mobile devices need to discover each other first in order to establish a proper radio connection for D2D communications, especially considering the possibility of semi-autonomous D2D setup. To facilitate this need, it has been proposed to have the mobile devices transmit some individual reference signals or sequences or to broadcast some predefined beacons, which are unique among a local networking neighborhood of the mobile devices (such as the cell area of the serving base station), in certain predefined channels.
Given a huge number of mobile devices or network nodes in recent or future mobile cellular systems employing D2D communications, an important problem is how to define and allocate limited cellular resources for reliable and effective transmission of such reference signals, sequences and/or beacons on physical channels in the context of D2D connection establishment. In the following, such resource allocation is referred to as initial resource allocation for supporting direct D2D communications.
According to a conceivable approach for such problem, it may be assumed that there are a certain number of designated beacon channels for D2D communications per cell and D2D mobile devices camping in a respective cell may select one channel for D2D beaconing. However, as indicated below, such approach is disadvantageous in that it addressed initial resource allocation for D2D communications on a cell basis and is based on a UE-centric distributed control.
Accordingly, there is a demand for a feasible solution for facilitating efficient (initial) resource allocation for direct terminal-to-terminal communication in a cellular system.